The nation has considerable resources invested in drinking water and sewer services. Although local governments are major investors in this sector, other capital intensive services (i.e., transportation, communication, and electricity) compete for limited local resources. Beyond providing safe drinking water and environmental protection, water and sewer investments also contribute to economic growth in the local and national economies.
Since 1970, total public investment in wastewater infrastructure and operating and maintenance expenses (O&M) exceeded $658.4 billion. According to ASCE, water and wastewater systems are the second largest public works infrastructure in the country. Infrastructure investment contributes to economic growth by expanding the productive capacity of a locality, region, state, or the nation as a whole. Public infrastructure investment yields positive returns, and investment in water and sewer infrastructure has greater returns than most other types of public infrastructure.
Local governments are the primary investors in water and sewer systems. Beyond investments in physical plant and equipment, spending on the operations and maintenance of water systems also is a major financial obligation for states and local governments. In 2004, spending by states and localities on water and sewer operations and maintenance was $51.2 billion, second only to highway and roads.
Stormwater.
Municipal water systems must address two different sources of urban drainage: stormwater and sewage. Stormwater is rainwater and melted snow that runs off streets, lawns, and other sites. When stormwater is absorbed into the ground, it is filtered and ultimately replenishes aquifers or flows into streams and rivers. In developed areas, however, impervious surfaces such as pavement and roofs prevent precipitation from naturally soaking into the ground. Traditional stormwater management design focuses on collecting stormwater in piped networks and transporting it off site as quickly as possible, either directly to a stream or river, to a large stormwater management facility (basin), or to a combined sewer system flowing to a wastewater treatment plant. Systems to manage stormwater are often called “storm drains” or “storm sewers.”
Sewage Systems.
A sewage system conveys wastewater to a publicly-owned treatment works (POTW). Two types of public sewer systems predominate in the United States: combined sewer systems (CSSs) and sanitary sewer systems (SSSs).
Combined Sewer Systems.
CSSs were among the earliest sewer systems constructed in the United States and were built until the first part of the 20th century. A CSS is defined as a wastewater collection system owned by a state of municipality that conveys domestic, commercial, and industrial wastewaters and storm water runoff through a single pipe system to a publicly-owned treatment works (POTW).
Sanitary Sewer Systems.
Eventually, local governments began to separate storm flows from wastewater flows and separate sanitary sewer and storm sewer systems replaced combined sewer systems in many areas. Since the first part of the 20th century, municipalities in the United States have generally constructed SSSs. An SSS is defined as a municipal wastewater collection system that conveys domestic, commercial, and industrial wastewater, and limited amounts of infiltrated groundwater and storm water, to a POTW. SSSs are not designed to collect large amounts of storm water runoff from precipitation events. Areas served by SSSs often have a municipal separate storm sewer system to collect and convey runoff from rainfall and snowmelt.
Design of Sewer Systems.
In the ideal case, a sewer system is completely gravity-powered. Pipes from each house or building flow to a sewer main that runs, for example, down the middle of the street. Typically, the sewer main might be 3 to 5 feet (1 to 1.5 m) in diameter. Periodically, a vertical access chamber (called a manhole) will run up from the main to the surface, where it is covered by a manhole cover. Manholes allow access to the main for maintenance purposes.
The sewer mains flow into progressively larger pipes until they reach the wastewater treatment plant. In order to help gravity do its job, the wastewater treatment plant is usually located in a low-lying area, and sewer mains will often follow creekbeds and streambeds (which flow naturally downhill) to the plant. Normally, gravity cannot do all the work and a sewer system will include a grinder-pump or a lift station to move the wastewater up over a hill.
Once the water reaches the wastewater treatment plant, it goes through one, two or three stages of treatment to allow solids to settle out of the water and for scum to rise; to remove organic materials and nutrients with the help of bacteria; and to remove phosphorous and nitrogen from the water, prior to discharge of the treated water.
Sewer Overflows.
During wet weather events (e.g., rainfall or snowmelt), the combined volume of wastewater and stormwater runoff entering CSSs often exceeds conveyance capacity. Most CSSs are designed to discharge flows that exceed conveyance capacity directly to surface waters, such as rivers, streams, estuaries, and coastal waters. Such events are called combined sewer overflows, or CSOs. A CSO is defined as the discharge from a CSS at a point prior to the POTW treatment plant. Some CSO outfalls discharge infrequently, while others discharge every time it rains. Overflow frequency and duration varies from system to system and from outfall to outfall within a single CSS. Because CSOs contain untreated wastewater and storm water, they contribute microbial pathogens and other pollutants to surface waters. CSOs can impact the environment and human health. Specifically, CSOs can cause or contribute to water quality impairments, beach closures, shellfish bed closures, contamination of drinking water supplies, and other environmental and human health problems.
New York City, Washington D.C., Seattle and other cities with combined sewer systems have this problem due to a large influx of storm water after every heavy rain. Some cities have dealt with this by adding large storage tanks to hold the combined sewer overflow discharge until it can be treated. Chicago and many other cities have a system of tunnels, collectively called the Deep Tunnel, underneath the city for storing its combined sewer overflow discharge.
About 772 communities in the United States have combined sewer systems, serving about 40 million people. CSO discharges during heavy storms can cause serious water pollution problems in these communities. Pollutants from CSO discharges can include bacteria and other pathogens, toxic chemicals, and debris. The U.S. Environmental Protection Agency (EPA) issued a policy in 1994 requiring municipalities to make improvements to reduce or eliminate CSO-related pollution problems. In 2000, Congress amended the Clean Water Act to require the municipalities to comply with the EPA policy.
Untreated or partially treated discharges from SSSs are commonly referred to as sanitary sewer overflows, or SSOs. SSOs have a variety of causes including blockages, line breaks, sewer defects that allow excess storm water and groundwater to overload the system, lapses in sewer system operation and maintenance, inadequate sewer design and construction, power failures, and vandalism. An SSO is defined as an untreated or partially treated sewage release from a SSS.
Generally speaking, SSOs can occur at any point in an SSS, during dry weather or wet weather. SSOs include overflows that reach waters of the United States. SSOs also include overflows out of manholes and onto city streets, sidewalks, and other terrestrial locations. A limited number of municipalities have SSOs that discharge from fixed points within their sewer system. SSSs can back up into buildings, including private residences. When sewage backups are caused by problems in the publicly owned portion of an SSS, they are considered SSOs.
SSOs can range in volume from one gallon to millions of gallons. The microbial pathogens and other pollutants present in SSOs can cause or contribute to water quality impairments, beach closures, shellfish bed closures, contamination of drinking water supplies, and other environmental and human health problems.
Aging sewer line infrastructure in many communities allows rain and snow melt to enter sanitary sewer systems. During significant wet weather events it is possible for influent flows to exceed the treatment capacity of existing secondary treatment units. Known as “peak flows,” these wet weather flows are sometimes diverted around secondary treatment units and then either recombined with flows from the secondary treatment units or discharged directly into waterways from the treatment plant in order to prevent any damage to the treatment facility. Operators of wastewater treatment plants must manage these high flows to both ensure the continued operation of the treatment process and to prevent backups and overflows of raw wastewater in basements or on city streets.
SSOs can also damage property and the environment. When basements flood, the damaged area must be thoroughly cleaned and disinfected to reduce the risk of disease. Cleanup can be expensive for homeowners and municipalities. Rugs, curtains, flooring, wallboard panels, and upholstered furniture usually must be replaced.
A key concern with SSOs that enter oceans, bays, estuaries, rivers, lakes, streams, or brackish waters is their effect on water quality. When bodies of water cannot be used for drinking water, fishing, or recreation, society experiences an economic loss. Tourism and waterfront home values may fall. Fishing and shellfish harvesting may be restricted or halted. SSOs can also close beaches. One 1994 study claims that SSOs closed beaches across the nation that year for a total of more than 300 days.
Mitigation of Sewer Overflows.
Municipalities in the United States have been undertaking projects to mitigate CSOs since the 1990s. For example, prior to 1990, the quantity of untreated combined sewage discharged annually to lakes, rivers and streams in southeast Michigan was estimated at more than 30 billion US gallons (110,000,000 m3) per year. In 2005 with nearly $1 billion of a planned $2.4 billion CSO control investment put into operation, untreated discharges have been reduced by more than 20 billion US gallons (76,000,000 m3) per year. This investment that has yielded a 67% reduction in CSO has included numerous sewer separation, CSO storage and treatment facilities, tunnels, and wastewater treatment plant improvements constructed by local and regional governments. Many other areas in the United States are undertaking similar projects.
Many avoidable SSOs are caused by inadequate or negligent operation or maintenance, inadequate system capacity, and improper system design and construction. These SSOs can be reduced or eliminated by:                Sewer system cleaning and maintenance;        Reducing infiltration and inflow through system rehabilitation and repairing broken or leaking service lines;        Enlarging or upgrading sewer, pump station, or sewage treatment plant capacity and/or reliability;        Construction wet weather storage and treatment facilities to treat excess flows; and        Communities also should address SSOs during sewer system master planning and facilities planning, or while extending the sewer system into previously unsewered areas.        
The resulting negative impacts of CSOs and SSOs can, and have in some areas, been reduced with the design and construction of traditional deep tunnel systems for the conveyance and storage of sewage overflows. These systems are costly to design, construct, and maintain and provide limited tangible economic benefit to the municipality over the long term. For example, the Deep Tunnel project (the Tunnel and Reservoir Plan (abbreviated TARP)) in Chicago was commissioned in the mid-1970s to reduce flooding in the metropolitan Chicago area, and to reduce the harmful effects of flushing raw sewage into Lake Michigan by diverting storm water and sewage (CSOs) into temporary holding reservoirs. The Deep Tunnel system is designed to be a network of 109.4 miles of tunnels of varying size. Completion of the system is not anticipated until 2029, but substantial portions of the system have opened and are operational. Over the 30 years of construction, more than $3 billion has been spent on the project.
Sewage tunnels such as those in Deep Tunnel are normally constructed very deep (100 ft. or deeper) below the normal terrain, below the points of sewage overflow. The depth of the tunnel is dependent on local geologic conditions. Deep and expensive “drop shafts” are constructed along the tunnel route so that the sewage overflows can be routed into the tunnel. Often “consolidation sewers” are constructed at the surface to consolidate the number of overflows and direct them to the drop shafts. Expensive and proprietary tunnel boring machines (TBMs) are employed to bore the tunnels. A tunnel boring machine (TBM) also known as a “mole”, is a machine used to excavate tunnels with a circular cross section through a variety of soil and rock strata. Tunnel boring machines are used as an alternative to drilling and blasting (D&B) methods in rock and conventional “hand mining” in soil. TBMs have the advantages of limiting the disturbance to the surrounding ground and producing a smooth tunnel wall which significantly reduces the cost of lining the tunnel, and makes them suitable to use in heavily urbanized areas. The major disadvantage is the upfront cost. TBMs are expensive to construct, and can be difficult to transport. Tunnel spoils (excess rock and soil material) must then be brought to the surface and hauled to disposal sites at great cost. Depending on the local geologic conditions, once bored, the tunnels are often lined with concrete to create a watertight conduit.
Due to their depth, tunnels typically require high pressure (head) pumping stations to lift tunnel contents to the POTW. High head pumping stations typically have very high energy consumption and contribute to greenhouse gas emissions.
Some U.S. cities have undertaken sewer separation projects—building a second piping system for all or part of the community. In many of these projects, cities have been able to separate only portions of their combined systems. High costs or physical limitations may preclude building a completely separate system. In 2011 Washington, D.C. separated its sewers in four small neighborhoods at a cost of $11 million.